1. Field of the Invention
This invention relates generally to apparatus and procedures for detecting and characterizing cracks in structures, and more particularly to a sensitive method for detecting and characterizing cracks on the surfaces of piping and equipment of nuclear power plants and on surfaces of other systems and equipment where integrity of the structure is of great importance.
2. Description of the Prior Art
Structural crack detection and characterization is becoming an increasingly important problem in the nuclear power industry. A nuclear power plant contains numerous fluid-carrying components which must be welded together to ensure leak-proof containment of the fluids therein. These components include reactor pumps, valves, piping, piping components and pressure vessels. Flaws or cracks in these components can result from a variety of causes including fabrication errors, cyclic fatigue loads and environmental stress corrosion. Because of the intense heat generated by the welding process combined with environmental factors and stress, cracks sometimes develop in an approximately one-quarter inch zone near the weld, called the heat-affected zone. These cracks can range from no more than a few thousandths of an inch, comprising mere scratches, to much deeper cracks which can grow and lead to eventual structural failure. If cracking is suspected in essential reactor cooling pressure boundaries, substantial non-destructive examination must be performed to allow continued operation or to determine if repairs or replacement of the damaged components are needed. The cost of these inspections and repairs along with replacement power costs is very great. It is believed that a significant portion of these costs could be eliminated if reliable methods were available for early crack detection and characterization.
Various patents have been granted for methods and apparatus for crack detection. Typically, these methods require an external radiation source or input excitation. For example, U.S. Pat. No. 3,934,457 and 3,988,922 issued to Clark, et al. employ conventional ultrasonic examination devices and are limited in their ability to fully detect and characterize cracking. U.S. Pat. Nos. 4,169,758 issued to Blackstone, et al. is similarly limited to an ultrasonic detection apparatus and method.
Present nondestructive examination (NDE) methods of crack detection such as ultrasonic testing, eddy current inspection and acoustic emission, have been only partially successful in locating and establishing the extent of cracking.